The error is as follows:
WARN TermDocumentDeSerializationUtil Serialization error: Term could not be serialized! WARN TermDocumentDeSerializationUtil Serialization error: Document could not be serialized! ERROR DocumentBufferedFileStoreDataCellFactory Could not create DocumentBufferedFileStoreCell for document: [...] WARN RearrangeColumnsTable$ConcurrentNewColCalculator Unhandled exception in processFinished ERROR Strings To Document Execute failed: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cell at index 0 is null!
which is... not very helpful. From the circumstances I can only deduce that something must have gone wrong while parsing the content, as all other fields are unchanged from several thousand working iterations. The content strings come from xml processors, and as these crash easily, the strings should be quite "clean". I can't see anything obvious like missing values, either. It might be something unexpected like an overly long sentence producing an overflow, as I strip a lot of punctuation in preprocessing (not the ideal usage of this node, but it gives me a little bit more control).
I can live with one document less, so it's not a major problem for me, but this looks like something bad. I don't know if the initial error is justified, but if yes, the handling might be improved. Maybe give us missing values?